There are many potential applications for visible lasers, such as display, optical storage reading/writing, laser printing, and short-haul telecommunications employing plastic optical fibers (T. Ishigure et al., Electronics Letters, Mar. 16, 1995, Vol. 31, No. 6). For these applications, laser sources that are cost effective, moderately powerful, and that span the visible spectrum are desired. For infrared applications, the requirements for cost effectiveness and power have been well met by semiconductor laser diodes. However, in spite of the worldwide efforts of many industrial and academic laboratories, much work remains to be done to create viable laser diodes that produce light output that spans the visible spectrum, with the problem being especially acute in the green spectral region. Visible solid-state lasers using standard laser crystals (such as Nd:YAG) and frequency-doubling have been developed, but have not achieved the low-cost criteria. These lasers are also unable to provide the selection of arbitrary visible wavelengths that are desired. The same problems exist for gas lasers, along with the additional problem of extremely low wallplug efficiency.
In an effort to produce visible wavelength lasers, it would be advantageous to abandon inorganic-based systems and focus on organic-based laser systems, since organic-based gain materials can enjoy a number of advantages over inorganic-based gain materials in the visible spectrum. Traditional dye lasers, for example offer a very wide range of wavelengths. However, liquid dye lasers have not proven to be feasible in non-laboratory applications.
Other organic-based gain materials have the beneficial properties of low unpumped scattering/absorption losses and high quantum efficiencies. In comparison to inorganic laser systems, organic lasers are relatively inexpensive to manufacture, can be made to emit over the entire visible range, can be scaled to arbitrary size and, most importantly, are able to emit multiple wavelengths (such as red, green, and blue) from a single chip. Over the past number of years, there has been increasing interest in making organic-based solid-state lasers. The laser gain material has been either polymeric or small molecule and a number of different resonant cavity structures were employed, such as, microcavity (Kozlov et al., U.S. Pat. No. 6,160,828 issued Dec. 12, 2000, titled “Organic Vertical-Cavity Surface-Emitting Laser”), waveguide, ring microlasers, and distributed feedback (see also, for instance, G. Kranzelbinder et al., Rep. Prog. Phys. 63, 729 (2000) and Diaz-Garcia et al., U.S. Pat. No. 5,881,083 issued Mar. 09, 1999, titled “Conjugated Polymers As Materials For Solid State Laser”). Kozlov also disclosed an external cavity laser in U.S. Pat. No. 6,160,828. Koslov teaches an electrically pumped laser device in a cavity between two planar surfaces that includes an intracavity lens.
A problem with all of these structures is that in order to achieve lasing it was necessary to excite the cavities by optical pumping using another laser source. It is much preferred to electrically pump the laser cavities since this generally results in more compact and easier to modulate structures.
The external cavity embodiment disclosed by Kozlov in U.S. Pat. No. 6,160,828 is electrically pumped, however, the disclosed external cavity requires a transparent electrode deposited above the organic gain materials in order to operate as intended. Currently, the only viable electrically conductive transparent material in use is indium tin oxide (ITO), for which the deposition process commonly requires the substrate to be heated at an elevated temperature that would destroy most commonly used organic luminescent materials. Therefore, Kozlov's disclosed embodiment may prove difficult to manufacture.
A main barrier to achieving electrically-pumped organic lasers is the small carrier mobility of organic material, which is typically on the order of 10−5 cm2/(V-s). This low carrier mobility results in a number of problems. Devices with low carrier mobilities are typically restricted to using thin layers in order to avoid large voltage drops and ohmic heating. These thin layers result in the lasing mode penetrating into the lossy cathode and anode, which causes a large increase in the lasing threshold (Kozlov et al., Journal of Applied Physics, Vol. 84, No. 8, Oct. 15, 1998). Since electron-hole recombination in organic materials is governed by Langevin recombination (whose rate scales as the carrier mobility), low carrier mobilities result in orders of magnitude having more charge carriers than singlet excitons; one of the consequences of this is that charge-induced (polaron) absorption can become a significant loss mechanism (Tessler et al., Applied Physics Letters, Vol. 74, No. 19, May 10, 1999). Assuming laser devices have a 5% internal quantum efficiency, using the lowest reported lasing threshold to date of ˜100 W/cm2 (Berggren et al., Letters to Nature Vol. 389, Oct. 02, 1997), and ignoring the above mentioned loss mechanisms would put a lower limit on the electrically-pumped lasing threshold of 1000 A/cm2. Including these loss mechanisms would place the lasing threshold well above 1000 A/cm2, which to date is the highest reported current density, which can be supported by organic devices (Tessler et al., Advanced Materials, 1998, Vol. 10, No. 1).
One of the advantages of organic-based lasers is that since the gain material is typically an amorphous film, devices can be formed inexpensively when compared to lasers with gain materials that require a high degree of crystallinity (either inorganic or organic materials). Additionally, lasers based upon organic amorphous gain materials can be fabricated over large areas without regard to producing large regions of single crystalline material; as a result they can be scaled to arbitrary size resulting in greater output powers. Because of their amorphous nature, organic-based lasers can be grown on a wide variety of substrates; thus, materials such as glass, flexible plastics, and Si are possible supports for these devices. Thus there can be significant cost advantages as well as a greater choice in usable support materials for amorphous organic-based lasers. The usage of single crystal organic lasers would obviate all of these advantages.
An alternative to electrical pumping for organic lasers is optical pumping by incoherent light sources, such as, light emitting diodes (LEDs), either inorganic (McGehee et al., Applied Physics Letters, Vol. 72, No. 13, Mar. 30, 1998) or organic (Berggren et al., U.S. Pat. No. 5,881,089 issued Mar. 9, 1999, titled “Article Comprising An Organic Laser”). This possibility is the result of unpumped organic laser systems having greatly reduced combined scattering and absorption losses (˜0.5 cm−1) at the lasing wavelength, especially when one employs a host-dopant combination as the active media. Even taking advantage of these small losses, the lowest reported optically-pumped threshold density for organic lasers to date is 100 W/cm2 based on a waveguide laser design (Berggren et al., Letters to Nature, Vol. 389, Oct. 02, 1997). Since off-the-shelf inorganic LEDs can only provide up to ˜20 W/cm2 of power density, it is necessary to take a different route to enable optical pumping by incoherent sources. There are a few disadvantages to organic-based gain media, but with careful laser system design these can be overcome. Organic materials are sensitive to a variety of environmental factors like oxygen and water vapor. Efforts to reduce sensitivity to these variables typically result in increased device lifetime. Additionally, organic materials can suffer from low optical and thermal damage thresholds. Devices will have a limited pump power density in order to preclude irreversible damage to the device.
For this reason, generating high power from an organic laser poses a difficult problem. In order to generate a high power and remain within prescribed limits for the power density, it is necessary to use a very large area for the generation of the laser power. However, one must also maintain a coherent beam, which is difficult in a thin film structure.
When faced with a similar problem with inorganic semiconductors and solid-state lasers, it has been known to utilize a thin gain medium in an external resonator. For example, in U.S. Pat. No. 5,553,088 by Brauch et al., issued Sep. 03, 1996, titled “Laser Amplifying System,” a thin disk of optically-pumped solid laser material is situated on one end mirror of a standard laser resonator. The resonator can be designed to produce a single Gaussian laser mode with a relatively large area on the laser material in order to scale the laser power without damaging the laser material. A similar technique has also been known using vertically-emitting semiconductor laser structures. One version using multiple passes over the laser structure was disclosed in U.S. Pat. No. 5,131,002 by Mooradian, issued Jul. 14, 1992, titled “External Cavity Semiconductor Laser System.” More recently, more standard laser resonators have been disclosed with vertically-emitting semiconductor laser structures, both optically-pumped (U.S. Pat. No. 5,991,318 issued to Caprara et al. on Nov. 23, 1999, titled “Intracavity Frequency-Converted Optically-Pumped Semiconductor Laser,” and related patents) and electrically-pumped (U.S. Pat. No. 6,243,407 issued to Mooradian on Jun. 05, 2001, titled “High Power Laser Devices”).
A disadvantage that all these lasers share is the need to use nonlinear frequency conversion to generate visible laser light. This adds to the cost of the laser system due to the need for a nonlinear optical material and the appropriate phase-matching provisions. Furthermore, the resulting requirement for a stable single longitudinal mode is a disadvantage for display applications (in which a broad spectral width is desired for speckle suppression) and other applications that require wavelength tunability. Finally, the frequency conversion, especially in continuous-wave operation, reduces the efficiency of the laser, requiring more optical pump power (for the optically-pumped cases) and a larger mode area.
Another disadvantage faced by the optically-pumped external cavity lasers disclosed in '088, '002, and '318, is that the laser materials are characterized by threshold densities high enough to require another laser as the pump source. It would be desired to use less costly, incoherent sources such as light-emitting-diodes for optical pumping should the threshold densities become low enough for practical usage of light emitting diodes. A third disadvantage of the above materials is that they are all crystalline and grown on flat substrates.
There is a need for a good quality laser beam with high-power operation, laser mode control, and tunability in an organic-based laser structure capable of excitation with incoherent light sources such as LEDs.